The subject matter of the invention relates to an arrangement in a cascade refrigeration system with screw compressors having a low-temperature circuit and a high-temperature circuit connected with each other thermally via a heat exchanger wherein the refrigerant of the low-temperature circuit is condensed in said heat exchanger, and the refrigerant of the high-temperature circuit is expanded in said heat exchanger. In the evaporator portion of the high-temperature circuit, the energy from the evaporator of the low-temperature circuit and the input power reduced by an oil cooling capacity are removed.
According to prior art, the high-temperature circuit in such systems operates at an evaporating temperature lower than the condensing temperature of the low-temperature system. The refrigerant to be condensed of the low-temperature circuit features a relatively high superheat with resulting relatively great temperature differences occurring in the heat exchanger mentionend above.
A disadvantage of the prior art is that with the system mentioned in the known arrangement a higher energy consumption on the high-temperature side is required for removal of the heat quantity from the low-temperature circuit as refrigeration on the high-temperature side with relation to the outlet temperature of the refrigerant on the low-temperature side is generated at a temperature at which later on condensation of the refrigerant has to take place on the low-temperature side, usually 2 to 5 Kelvin below the condensing temperature of the low-temperature circuit. Thus, generation of refrigeration on the high-temperature side for heat removal from the low-temperature circuit is uneconomical and can be improved.
The object of the invention is to remove part of the superheat from the process of the low-temperature refrigeration system at another evaporating temperature level.
The feature of the invention is that in addition to the heat exchanger mentioned in which the refrigerant of the low-temperature side is condensed, and the refrigerant of the high-temperature side is expanded, a second heat exchanger is arranged in flow direction ahead of the said heat exchanger on the side of the refrigerant to be condensed which is fed by liquid refrigerant from the high-temperature circuit for desuperheating the fluid flow from the low-temperature circuit. This partial refrigerant flow of the high-temperature circuit evaporating as a result will be supplied to the economizer opening on the screw compressor of the high-temperature refrigeration system at which the inlet pressure is higher than the pressure on the suction side of the screw compressor.
The advantage of this technical solution is that the coefficient of performance of the entire system will be improved by 5 to 10%, and hence 5 to 10% of the energy cost will be saved, and the economic efficiency of such system will be improved considerably as a result. The advantage is due to the fact that a portion of the heat is removed from the low-temperature circuit at a higher evaporating temperature at which the Carnot efficiency considerably exceeds the Carnot efficiency at which condensation of the refrigerant takes place in the low-temperature circuit. A further advantage is that due to this arrangement the suction flow rate of the refrigerant compressor on the high-temperature side can be reduced by 10 to 20% with a resulting reduction of the cost of the refrigeration system on the high-temperature side.